In concentrator photovoltaics (CPV), the directly incident solar radiation is typically focused onto a solar cell by concentrator optics, such that the intensity of irradiation on the cell is higher by the so-called concentration factor. There are several optical approaches to concentrator optics, which are usually based on refraction, reflection or total internal reflection at specifically shaped optical components (P. Benitez and J. C. Minano, “Concentrator optics for the next-generation photovoltaics”, in A. Marti and A. Luque (Ed.), “Next Generation Photovoltaics”, Institute of Physics Publishing, Series in Optics and Optoelectronics, Bristol and Philadelphia, ISBN 0750309059, 2004). With highly concentrating systems, i.e. with a concentration factor>50, it is also common to effect optical concentration in two stages, by a primary and a secondary concentrator. The secondary concentrator in turn typically includes various features utilizing the above mentioned optical effects. It can serve to increase the concentration, to enlarge the angular range at which the solar cell receives radiation and to distribute it more homogenously over the cell area. In homogenization, it is also possible to influence the cross-section of the ray beam. In particular, those solar cells that are highly efficient may be suited as thereby the technical requirements for optical concentration and for the linked tracking of the system can be justified. These can be high-efficient Si solar cells, e.g., back-contact solar cells, but also monolithically interconnected stacked solar cells on the basis of III-V semiconductor materials (e.g., multi-junction cells, MJC). Multilayer structures of III-V-compound semiconductors are typically grown epitaxially. A typical structure of a triple cell includes a germanium basic cell, a GAInAs center cell and a GaInP top cell. Germanium wafers are the substrates onto which the thin III-V semiconductor layers are typically deposited. The basic material as well as the manufacture of MJC can be cost-intensive, which is why it is generally assumed that a very high concentration, i.e. small cell areas based on the solar aperture, are preferable for the cost effectiveness of the CPV (C. Algora, “The importance of the very high concentration in third-generation solar cells”, in A. Marti and A: Luque (Ed.), “Next Generation Photovoltaics”, Institute of Physics Publishing, Series in Optics and Optoelectronics, Bristol and Philadelphia, ISBN 0750309059, 2004).
Here, a problem can be utilizing, as efficiently as possible, cost-intensive wafers with very high-efficient solar cells for concentrator photovoltaics to permit an economical utilization of this technology.
Existing technology typically provides for processing the high-efficient solar cells on wafers and subsequently separating them into rectangular, in most cases square, chips by sawing. The individual chips then typically have edge lengths of 1 to 10 mm.
Many of the employed optical primary concentrators generate a largely rotationally symmetric focus. Without a secondary concentrator, the usable active area of the concentrator solar cells is therefore also rotationally symmetric. The corners of the cell, which is in this case square are used as bonding areas for the electrical interconnection of the cell. However, the use of the expensive wafer material with a rectangular, in particular, square geometry and a circular active cell area is altogether not satisfactory. Including area loss by saw cutting and the wafer edge, hardly more than 60% of the wafer area can be utilized as active solar cell area. With cell sizes below 2 mm, the usable area is typically drastically reduced due to cutting waste.
A secondary concentrator can distribute the radiation more homogenously onto a rectangular output face like a kaleidoscope by multiple reflection. Such secondary concentrators are described in literature (J. M. Gordon, “Concentrator optics”, in A. Luque and V. Andreev (Eds.), Concentrator Photovoltaics, Springer Series in Optical Sciences 130, Springer-Verlag, Berlin Heidelberg (2007), and US 2008/0087323 A1), and can accordingly include a circular entrance and a nearly rectangular emergence surface or rectangular surfaces. However, these secondary concentrators are preferably lossless with a relatively high width-height ratio, i.e. they preferably include very high reflectance and, in case of massive systems, they are preferably made of a transparent material of very low absorption. Currently, such secondary concentrators are typically only manufactured at very high costs.
Currently, inexpensive solutions with high wafer utilization are not yet known.